The present invention relates to a four-cycle engine for a motor vehicle designed to use so-called xe2x80x9cinternalxe2x80x9d exhaust gas recirculation (EGR) in a positive way by causing part of already combusted exhaust gases to remain in a combustion chamber.
EGR is a widely known conventional technique in which an exhaust air line and an intake air line of an engine are connected by an external EGR passage and part of exhaust gases is returned to the intake air line through the external EGR passage to reduce nitrogen oxides (NOx), as described in Japanese Unexamined Patent Publication No. 5-86988, for example. This Patent Publication discloses an external EGR system in which a specific amount of cooled exhaust gas (EGR gas) is re-introduced into a combustion chamber through an external EGR passage which is provided with an EGR cooler and the recirculation rate (EGR rate) is increased in higher-load ranges of the engine, so that NOx emissions are decreased and increases in combustion temperature and exhaust gas temperature are avoided in the higher-load ranges. Avoiding the increases in combustion temperature and exhaust gas temperature is advantageous for improving combustion efficiency, preventing knocking combustion, and for improving the reliability of exhaust gas converting catalysts.
Internal EGR is another form of exhaust gas recirculation technique of the prior art, in which already combusted exhaust gases are intentionally caused to remain in the combustion chamber instead of recirculating the exhaust gases through the external EGR passage. A common approach used for internal EGR is to increase the amount of overlap between open periods of intake and exhaust valves. For this purpose, an internal EGR system is usually provided with a valve timing adjuster for varying opening and closing timing of the intake and exhaust valves. The valve timing adjuster increases the amount of residual exhaust gas left in the combustion chamber by causing xe2x80x9cblowing-backxe2x80x9d of the exhaust gas during overlap periods, for instance, by increasing the amount of overlap in a range requiring the internal EGR.
An arrangement for adjusting the opening and closing timing of the intake and exhaust valves in accordance with engine operating conditions is disclosed in Japanese Unexamined Patent Publication No. 10-266878. According to the arrangement of this Patent Publication, the exhaust valve is closed at a point before the top dead center on an intake stroke and the intake valve is opened at a point after the top dead center on the intake stroke to cause multi-point self-ignition in the combustion chamber and to reduce pumping loss in a specific low-load operating range of the engine and, as required torque decreases, the closing point of the exhaust valve is advanced and the opening point of the intake valve is retarded.
Among the aforementioned conventional arrangements, the external EGR system shown in Japanese Unexamined Patent Publication No. 5-86988 requires that intake and exhaust systems of the engine be provided with such additional facilities as the external EGR passage, an EGR valve and the EGR cooler. While a specified portion of exhaust emissions is returned to an intake passage on a downstream side of downstream of a throttle valve and re-introduced into the combustion chamber in the engine provided with the external EGR system, it is difficult to recirculate a sufficient amount of exhaust gas under higher-load conditions in a high-speed range, because negative pressure in the intake passage decreases on the downstream side of the throttle valve and an intake period of each cylinder shortens under high-speed high-load conditions. In addition, the external EGR system is associated with a problem that exhaust gas deposits are likely to accumulate in the intake air line.
On the other hand, although the aforementioned internal EGR approach enables exhaust gas recirculation by increasing the amount of overlap between the open periods of the intake and exhaust valves, eliminating the need for the external EGR passage, it is difficult to avoid increases in combustion temperature and exhaust gas temperature in higher-load ranges because the temperature of the residual exhaust gas left in the combustion chamber is high. Furthermore, if the overlap period is fixed, it is substantially shortened and it becomes difficult to achieve sufficient internal EGR due to a decrease in effective valve opening period when the engine speed increases. Accordingly, it is necessary to significantly increase the overlap period in high-speed ranges to ensure sufficient effects of internal EGR up to the high-speed ranges. The valve opening and closing timing should be made adjustable within a wide variable range to meet this requirement, and this makes the construction of the valve timing adjuster complex. If the overlap period is increased, however, both the intake and exhaust valves would be relatively widely opened at the intake top dead center, and this makes it necessary to form a deep recess in a top surface of a piston to avoid interference between the intake and exhaust valves. This recess in the piston could exert an adverse effect on combustion in the combustion chamber.
The arrangement disclosed in Japanese Unexamined Patent Publication No. 10-266878 is intended to cause self-ignition by decomposing and radicalizing fuel in a mixture supplied into the combustion chamber with the aid of high-temperature exhaust gas, and not to avoid increases in combustion temperature and exhaust gas temperature in higher-load ranges.
In view of the foregoing, it is an object of the invention to provide an automotive four-cycle engine capable of producing sufficient effects of EGR and avoid increases in combustion temperature and exhaust gas temperature by internal EGR up to medium- to high-speed high-load ranges.
In one principal feature of the invention, an automotive four-cycle engine whose power cycle has four strokes including intake, compression, expansion and exhaust strokes, each stroke being defined as a movement of a piston in a single direction between a top dead center and a bottom dead center, is constructed such that a closing point of an exhaust valve defined as a point of transfer from an acceleration portion to a constant speed portion on its valve lift characteristics curve is set to a point a specific period before the intake top dead center between the exhaust stroke and the intake stroke and an opening point of an intake valve defined as a point of transfer from a constant speed portion to an acceleration portion on its valve lift characteristics curve is set to a point after the intake top dead center at least in medium- to high-speed ranges at least in medium- to high-load regions of the engine excluding its maximum-load region.
In this construction of the invention, the internal EGR is accomplished with already combusted gases left in a combustion chamber as the exhaust valve is closed at the point the specific period before the intake top dead center in the medium- to high-speed ranges in the medium- to high-load regions of the engine. Since the intake valve is opened after the intake top dead center, the pressure in the combustion chamber during the period from the closing point of the exhaust valve to the intake top dead center and, as a consequence, heat is sufficiently dissipated during this period when the temperature in the combustion chamber before the temperature decreases due to subsequent pressure drop after the intake top dead center. As the already combusted gases in a combustion chamber are cooled in this fashion, the effect of avoiding increases in combustion temperature and exhaust gas temperature is obtained as is the case with an external EGR system which re-introduces low-temperature exhaust gases.
According to the invention, it is preferable to set the opening point of the intake valve to a point a specific period after the intake top dead center within an earlier part of the intake stroke and provide a period during which the intake top dead center occurs and both the exhaust valve and the intake valve are closed at least in the medium-speed range at least in the medium- to high-load regions of the engine.
Preferably, the automotive four-cycle engine of the invention comprises an operating condition identifier for determining engine operating conditions based on engine speed and engine load, a valve timing adjuster capable of varying the opening and closing timing of at least the exhaust valve or the intake valve, and a controller which controls the valve timing adjuster to adjust the valve opening and closing timing according to the engine operating conditions in such a manner that the period from the closing point of the exhaust valve to the opening point of the intake valve, during which the intake top dead center occurs, becomes longer in the medium-speed range than in the high-speed range in the medium- to high-load regions of the engine when the engine operating conditions determined by the operating condition identifier indicate that the engine is at least in the medium- to high-load regions.
In this construction, the effect of progressive decreases in effective valve opening periods of the exhaust valve and the intake valve toward higher engine speeds is corrected by varying the valve timing, so that the same degree of internal EGR is obtained through the medium- to high-speed ranges in the medium- to high-load regions of the engine.
In the above construction, it is preferable to make a crank angle period from the intake top dead center to a closing point of the intake valve longer than a crank angle period from the closing point of the exhaust valve to the intake top dead center in the medium-speed range at least in the medium- to high-load regions of the engine.
Preferably, the automotive four-cycle engine of the invention comprises an operating condition identifier for determining engine operating conditions based on engine speed and engine load, a valve timing adjuster capable of varying the opening and closing timing of at least the exhaust valve, and a controller which controls the valve timing adjuster in such a manner that the closing point of the exhaust valve occurs after the intake top dead center when the engine operating conditions determined by the operating condition identifier indicate that the engine is in the maximum-load region.
In this construction, the amount of exhaust gas recirculated by the internal EGR is decreased and the amount of fresh air introduced is increased, so that sufficient engine output is produced.
In the automotive four-cycle engine of this construction, the valve timing adjuster may be of a type which varies the valve opening and closing timing by varying the phase of rotation of a cam shaft with respect to a crankshaft without altering a valve open period.
In one aspect of the invention, the closing point of the exhaust valve may be gradually retarded as the engine load approaches from that in an engine operating range in which the closing point of the exhaust valve is set to the point the specific period before the intake top dead center to that in the maximum-load region. This serves to smoothly increase the engine output.
In another aspect of the invention, the valve timing adjuster is of a type capable of individually varying the opening and closing timing of the exhaust valve and the intake valve, and both the closing point of the exhaust valve and the opening point of the intake valve are set to a point after the intake top dead center in the maximum-load region in the high-speed range of the engine. This is advantageous for ensuring that a sufficient amount of fresh air is introduced in the high-speed maximum-load range when the open period of the intake valve is relatively short.
When the automotive four-cycle engine of this invention is provided with a turbocharger, it is preferable that at least the closing point of the exhaust valve be set to a point before the intake top dead center in the maximum-load region in the medium- to high-speed ranges of the engine.
In this construction, the internal EGR is accomplished and increases in combustion temperature and exhaust gas temperature are avoided in the maximum-load region even in the medium- to high-speed ranges of the engine and, yet, a sufficient amount of fresh air is drawn in by turbocharging while the internal EGR is being executed. In particular, anti-knocking performance of the engine is increased since the increase in combustion temperature is avoided. As a result, it becomes possible to increase the engine output by turbocharging, a feature which has conventionally been unachievable due to knocking.
It is preferable for the automotive four-cycle engine provided with the turbocharger to further comprise an air-fuel ration controller for controlling the air-fuel ratio of a mixture in a combustion chamber and thereby make the air-fuel ratio equal to or larger than the stoichiometric air-fuel ratio in the maximum-load region in the medium- to high-speed ranges of the engine. Since this arrangement increases the anti-knocking performance of the engine by setting the closing point of the exhaust valve to a point before the intake top dead center in the maximum-load region in the medium- to high-speed ranges of the engine as described above, it becomes unnecessary to enrich the mixture for preventing knocking combustion. Thus, both engine output performance and fuel efficiency can be improved by increasing the air-fuel ratio up to the stoichiometric air-fuel ratio while introducing a sufficient amount of fresh air by turbocharging.
In one principal feature of the invention, an automotive four-cycle engine whose power cycle has four strokes including intake, compression, expansion and exhaust strokes, each stroke being defined as a movement of a piston in a single direction between a top dead center and a bottom dead center, is constructed such that a closing point of an exhaust valve is set to a point in a latter part of the exhaust stroke a specific period before the intake top dead center between the exhaust stroke and the intake stroke such that already combusted gases are left in a combustion chamber, and an opening point of an intake valve is set to a point after the intake top dead center such that the pressure in the combustion chamber increases during a specific period from the latter part of the exhaust stroke to the intake top dead center, at least in medium- to high-speed ranges at least in medium- to high-load regions of the engine excluding its maximum-load region.
In this construction of the invention, it is preferable to make a crank angle period from the intake top dead center to a closing point of the intake valve longer than a crank angle period from the closing point of the exhaust valve to the intake top dead center, so that the pressure in the combustion chamber after the intake top dead center gradually decreases during a longer period than the aforementioned specific period during which the pressure in the combustion chamber increases, in the medium-speed range at least in the medium- to high-load regions of the engine.
According to the aforementioned construction of the engine of the invention, it is possible to decrease NOx emissions in medium- to high-speed ranges at least in the medium- to high-load regions of the engine, improve combustion efficiency and fuel efficiency by decreasing combustion temperature, and improve the reliability and durability of an exhaust system by preventing an increase in exhaust gas temperature. The invention is also useful for preventing knocking combustion and reducing pumping loss.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings.